Actually....that's not really true. They will breed just as fast if not faster than the regular pest snails, that's why you get them for free.
But you never see most of them because they are in the substrate during the day and lots of them stay there during the night.
They are live bearing hermaphrodites, but at least theres no eggs to worry about.
They do more good than harm, like some shrimp they generally make up for the extra bio-load by cleaning up.
And like pest snails, they wont multiply and become a problem unless you have food source for them in the tank. If they suddenly multiply, you should probably start feeding your fish less.
They get to an OK size, around 1".